function require(url, callback) {
    var oScript = document.createElement('script');
    oScript.type = 'text/javascript';
    oScript.async = true;
    oScript.src = url;
    /* 
     ** script标签的onload和onreadystatechange事件 
     ** IE6/7/8支持onreadystatechange事件 
     ** IE9/10支持onreadystatechange和onload事件 
     ** Firefox/Chrome/Opera支持onload事件 
     */

    // 判断IE8及以下浏览器 
    var isIE = !-[1, ];
    if (isIE) {
        console.log('IE')
        oScript.onreadystatechange = function () {
            if (this.readyState == 'loaded' || this.readyState == 'complete') {
                callback();
            }
        }
    } else {
        // IE9及以上浏览器，Firefox，Chrome，Opera 
        oScript.onload = function () {
            callback();
        }
    }
    document.body.appendChild(oScript);
}